Light Emitting Diode (LED) light strings have become quite popular recently for Christmas holiday decorating. They are much more energy efficient than incandescent lighting that has been around for many years. Since both the LED and the more conventional incandescent mini-light operate at very low voltage—usually between 2.0 to 3.5 volts—they are wired in electrical series connection with approximately 35 to 50 lights in a light string. As with incandescent lighting, when an LED bulb burns out or otherwise fails, the entire series light string goes out. To prevent this, bypass shunts can be wired across each LED to continue current through the light string in the event of such a failure. Such shunts are often resistive shunts, such as disclosed in U.S. Pat. Application No. 2014/0042903.
The latest trend in pre-lit Christmas trees is two color trees. This is where the consumer may choose clear (white) lights or multi-color lights to illuminate the Christmas tree. Inside each LED housing 2 is two LED's 4, 6—one clear and one color—connected in anti-parallel, as shown in FIG. 1. Such anti-parallel LED pairs are provided in a series wired circuit so that all clear LED's (oriented in one direction) are connected in electrical series and all multi-colored LED's (oriented in an opposite direction) are so wired. Selecting which color the tree will illuminate is easy. This can be done by flipping a switch 8, as shown in FIG. 2.
A bridge rectifier 10 provides full wave low flicker rectification to the LEDs. When current flows in one direction, the clear lights illuminate. When the polarity is switched, the current flows in the opposite direction, illuminating the multi-colored LED's. Because the voltage to the multi-colored LED's average lower than the clear lights, a series resistor 12 is inserted in the multi-color circuit. This series resistor can be shunted by a rectifier diode (D1) 14 (FIG. 2) which also may have a series resistor in series with it. This means keeps the proper current flowing in the circuit.
While LED's claim a long life, they do sometimes fail. When they are connected anti-parallel as shown in FIG. 1, each LED protects the companion LED in that housing from reverse voltage breakdown. When an LED fails, it no longer can protect its companion LED and that LED is damaged, and both colored light strings go out.
Accordingly, it would be desirable to provide a circuit for anti-parallel LEDs which protects the companion LED of each anti-parallel pair from reverse voltage breakdown in the event of failure of one of the two LEDs, thereby allowing a light string formed of anti-parallel LEDs to remain lit.